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Register a new userStructure and kinematics of Type II Cepheids in the Galactic bulge based on near-infrared VVV data
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Added by
Vittorio Francesco Braga
Publication date
2018
fields
Physics
and research's language is
English
Authors
V.F. Braga
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Type II Cepheids (T2Cs) are radially pulsating variables that trace old stellar populations and provide distance estimates through their period-luminosity (PL) relation. We trace the structure of old stars in the Bulge using new distance estimates and kinematic properties of T2Cs. We present new NIR photometry of T2Cs in the bulge from the VVV survey. We provide the largest sample (894 stars) of T2Cs with JHKs observations that have accurate periods from the OGLE catalog. Our analysis makes use of the Ks-band time-series to estimate mean magnitudes and distances by means of the PL relation. To constrain the kinematic properties of our targets, we complement our analysis with proper motions (PMs) based on both VVV and Gaia DR2. We derive an empirical Ks-band PL relation that depends on Galactic longitude and latitude: Ks0=(10.66+-0.02)-(2.21+-0.03)*(logP-1.2)-(0.020+-0.003)*l+(0.050+-0.008)*|b| mag; individual extinction corrections are based on a 3D reddening map. Our targets display a centrally concentrated distribution, with solid evidence of ellipsoidal symmetry--similar to the RR Lyrae (RRL) ellipsoid--and a few halo outliers up to >100 kpc. We obtain a distance from the Galactic center of R0=8.46+-0.03(stat.)+-0.11(syst.) kpc. We also find evidence that the bulge T2Cs belong to a kinematically hot population, as the tangential velocity components (sigma vl*=104.2+-3.0 km/s and sigma vb=96.8+-5.5 km/s) agree within 1.2sigma. Moreover, the difference between absolute and relative PM is in good agreement with the PM of Sgr A* from VLBA measures. We conclude that bulge T2Cs display an ellipsoidal distribution and have kinematics similar to RRLs, which are other tracers of the old, low-mass stellar population. T2Cs also provide an estimate of R0 that agrees excellently well with the literature, taking account of the reddening law.
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The Galactic center (GC) is the densest region of the Milky Way. Variability surveys towards the GC potentially provide the largest number of variable stars per square degree within the Galaxy. However, high stellar density is also a drawback due to blending. Moreover, the GC is affected by extreme reddening, therefore near infrared observations are needed. We plan to detect new variable stars towards the GC, focusing on type II Cepheids (T2Cs) which have the advantage of being brighter than RR Lyrae stars. We perform parallel Lomb-Scargle and Generalized Lomb-Scargle periodogram analysis of the $K_s$-band time series of the VISTA variables in the Via Lactea survey, to detect periodicities. We employ statistical parameters to clean our sample. We take account of periods, light amplitudes, distances, and proper motions to provide a classification of the candidate variables. We detected 1,019 periodic variable stars, of which 164 are T2Cs, 210 are Miras and 3 are classical Cepheids. We also found the first anomalous Cepheid in this region. We compare their photometric properties with overlapping catalogs and discuss their properties on the color-magnitude and Bailey diagrams. We present the most extensive catalog of T2Cs in the GC region to date. Offsets in E($J-K_s$) and in the reddening law cause very large ($sim$1-2 kpc) uncertainties on distances in this region. We provide a catalog which will be the starting point for future spectroscopic surveys in the innermost regions of the Galaxy.
In a step toward understanding the origin of the Galactic Halo, we have reexamined Type II Cepheids (T2C) in the field with new input from the second data release (DR2) of Gaia. For 45 T2C with periods from 1 to 20 days, parallaxes, proper motions, and [Fe/H] values are available for 25 stars. Only 5 show [Fe/H] < -1.5, while the remaining stars show thick disk kinematics and [Fe/H] > -0.90. We have compared the T2C stars of the field with their cousins in the globular clusters of the Halo and found that the globular clusters with T2C stars show metallicities and kinematics of a pure Halo population. The globulars may have formed during the overall collapse of the Galaxy while the individual thick disk T2C stars may have been captured from small systems that self-enriched prior to capture. The relationship of these two populations to the micro-galaxies currently recognized as surrounding the Galaxy is unclear.
We present the near-infrared observations of population II Cepheids in the Galactic bulge from VVV survey. We identify 340 population II Cepheids in the Galactic bulge from VVV survey based on their match with OGLE-III Catalogue. The single-epoch $JH$ and multi-epoch $K_s$ observations complement the accurate periods and optical $(VI)$ mean-magnitudes from OGLE. The sample consisting of BL Herculis and W Virginis subtypes is used to derive period-luminosity relations after correcting mean-magnitudes for the extinction. Our $K_s$-band period-luminosity relation, $K_s = -2.189(0.056)~[log(P) - 1] + 11.187(0.032)$, is consistent with published work for BL Herculis and W Virginis variables in the Large Magellanic Cloud. We present a combined OGLE-III and VVV catalogue with periods, classification, mean magnitudes and extinction for 264 Galactic bulge population II Cepheids having good-quality $K_s$-band light curves. The absolute magnitudes for population II Cepheids and RR Lyraes calibrated using Gaia and Hubble Space Telescope parallaxes, together with calibrated magnitudes for Large Magellanic Cloud population II Cepheids, are used to obtain a distance to the Galactic center, $R_0=8.34pm0.03{mathrm{(stat.)}}pm0.41{mathrm{(syst.)}}$, which changes by $^{+ 0.05}_{-0.25}$ with different extinction laws. While noting the limitation of small number statistics, we find that the present sample of population II Cepheids in the Galactic bulge shows a nearly spheroidal spatial distribution, similar to metal-poor RR Lyrae variables. We do not find evidence of the inclined bar as traced by the metal-rich red-clump stars. The number density for population II Cepheids is more limited as compared to abundant RR Lyraes but they are bright and exhibit a wide range in period that provides a robust period-luminosity relation for an accurate estimate of the distance to the Galactic center.
Planetary nebulae (PNe) are powerful tracers of evolved stellar populations. Among the 3000 known PNe in the Galaxy, about 600 are located within the 520 square-degree area covered by the VVV survey. The VVV photometric catalogue provides an important new dataset for the study of PNe, with high-resolution imaging in five near-infrared bands. Aperture photometry of known PNe in the VVV area was retrieved from source catalogues. Care was taken to minimise any confusion with field stars. The colours of the PNe we are determined for H-Ks, J-H, Z-Y, and Y-J, and compared to stars and to other types of emission line objects. Cloudy photo-ionisation models were used to predict colours for typical PNe. We present near-infrared photometry for 353 known PNe. The best separation from other objects is obtained in the H-Ks vs. J-H diagram. We calculated the emission-line contribution to the in-band flux based on a model for NGC 6720: we find that this is highest in the Z and Y bands at over 50%, lower in the J band at 40%, and lowest in the H and Ks bands at 20%. A new view of PNe in the wavelength domain of the Z and Y bands is shown. Photo-ionisation models are used to explore the observed colours in these bands. The Y band is shown to be dominated by HeI 1.083 mu and HeII 1.012 mu, and colours involving this band are very sensitive to the temperature of the ionizing star. The VVV survey represents a unique dataset for studing crowded and obscured regions in the Galactic plane. The diagnostic diagrams presented here allow one to study the properties of known PNe and to uncover objects not previously classified.
Near-IR data of Classical Novae contain useful information about the ejected gas mass and the thermal emission by dust formed during eruption, and provide independent methods to classify the objects according to the colour of their progenitors, and the fading rate and features seen after eruption. The VISTA Variables in the Via Lactea survey (VVV) is a near-IR ESO Public Survey mapping the MW bulge and southern plane. Data taken during 2010-2011 covered the entire area in the JHKs bands plus some epochs in Ks-band of the ongoing VVV variability campaign. We used the novae list provided by VSX/AAVSO catalogue to search for all objects within the VVV area. We used the VVV data to create a near-IR catalogue of the known Galactic novae in the 562 sq.deg. area covered by VVV. The VVV near-IR catalogue of novae contains JHKs photometry of 93 objects completed as of December 2012. We also checked in the ongoing VVV variability campaign for the light-curves of novae that erupted in the last years. VVV images can also be used to discover and study novae by searching for the expanding shell. Since objects are seen at different distances and reddening levels, the colour-magnitude and colour-colour diagrams show the novae spread in magnitude as well as in colour. Dereddened colours and reddening-free indices were used with caution and cannot be a good approach in all cases since the distance and spectral features prevent more conclusive results for some extreme objects. Light-curves for some recent novae are presented. Thanks to its high spatial resolution in the near-IR, and large Ks-range, the VVV survey can be a major contributor for the search and study of novae in the most crowded and high-extinction regions of the Milky Way. The VVV survey area contains ~35 of all known novae in the Galaxy.
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